"Speaking of which, ..." is used as something like "oh, while we're on the topic, ...". I guess technically you could say "Speaking of that, ...", and be understood, but it'd sound really weird because "speaking of which" is a fixed expression in the English language, kinda like an idiom. Hope that helps!

Thank you Jeanna.

No, “Speaking of which” is technically an idiom— a phrase that doesn’t make sense on its own, but culturally carries a different meaning. For this reason, you cannot replace “which” with “that”, simply because “Speaking of which” is a complete phrase on its own. If you’d like to dive deeper (another idiom!) into idioma and common phrases, book a general English lesson with me and we will practice more!